Process of bleaching cellulosic material



Patented Oct. 9, 1923.

UNITED STATES PATENT OFFICE.

ELWOOD EBIE AND GEORGE A RICHTER, OF BERLIN, NEW HAMPSHIRE, ASSIGNORS 'IO BROWN COMPANY, OF BERLIN, NEW HAMPSHIRE, A CORPORATION OF MAINE.

PROCESS 'OF BLEACHING CELL'ULOSIC MATERIAL.

I No Drawing.

GEORGE- A. RICHTER, both citizens of, the

United States, residing at Berlin, in the county of Coos and State of New Hampshire, have invented new and useful Improvements in Processes of Bleaching-Cellulosic Material, of which the following is a specification.

The object of the present invention is to provide a method for super-bleaching bleached cellulosic pulp by. which such bleached pulp may be further bleached and whitened to an extent not heretofore possible, all Without loss in strength and with out affecting other desirable characteristics of the material.

While we shall describe a process embodying our invention as practiced in connection with sulphite pulp, it will be understood that-the process may be practiced with other forms of cellulosic materials if desired.

Ordinarily paper pulp or cellulosic pulp is bleached by the use of an alkaline hypochlo- ,rite solution, and, in carrying on this operation, it is necessary either to sacrifice strength and other desirable qualities of the pulp to attain a desired degree of whiteness, or else to sacrifice the whiteness of the stock in order'to preserve the strength and other desirable qualities in the pulp. As indicating more particularly What we have in mind as the other desirable characteristics of the cellulosic material, particularly of sulphite pulp for example, it is known that the bleaching operation affects the proportion in which cellulose of the order of alpha, beta and gamma cellulose are found in the finished stockfl An excess of beta cellulose in the stock results in a loss in strength of paper produced from the stock, and this is likewise true of paper formed of stock in which there is an excess of gamma cellulose.

On the other hand, however, it is desirable that the stock should contain a certain proportion of beta cellulose and possibly gamma cellulose, as, since alpha or resistant cellu lose, is hydrated with more difliculty than either beta or gamma cellulose, the presence of certain proportions of beta and possibly Application filed November 17, 1921 Serial No. 515,899.

gamma cellulose in the stock makes possible the hydration of the entire stock during the beating operation in a shorter time and with greater case than if the stock consisted solely of alpha cellulose. The degree of shrinkage of a sheet of paper during drying, formed of a given. stock, is dependent upon the proportions of the beta and gamma cellulose to the alpha cellulose in the initial stock, since the shrinkage in- -creases with the increase in proportion of beta and gamma cellulose, lso that it is desirable that there should not be excessive quantities of the latter forms of cellulose in the bleached stock. Over, or excess bleaching undulyincreases the proportions in the finishedproduct of these two forms of cellulose.

For indicating more or less roughly the proportions of alpha cellulose to beta and gamma cellulose in the bleached stock, it is customary to ascertain the copper number of such stock. Speaking generally, the copper number represents a semi-quantitative measure of oxidation products resulting from the action of the bleach or other oxidants on sulphite pulp. The more severe the bleaching action, the higher is the final copper number due to the increased proportion of those constituents responding to the beta and gamma cellulose determination. A normally bleached sulphite pulp shows a copper number of approximately 3 to 3.5. An in-- creased copper number in the sulphite pulp is indicative of a high percentage of shrinkage during drying of the stock, a high color, low strength and excessive slowness of the stock when beaten. Thus it will be seen that in the bleaching of pulp, if the bleaching reaction be carried on to a point where a very White color in the final product is secured, there are losses in other qualities which are necessary or desirable for the production of a fine'paper; and, on the other hand, where the bleaching reaction is stopped to preserve the desirable characteristics of the stock, the stock itself hasnot obtained its highest degree of whiteness. The color of bleached pulp is measured by a colorimeter, the vanations in color, according to given standards, 1

being. designated arbitrarily by numbers in a certain-order of progression from 90 to 105.

A normally well bleached pulp, possess1ng the desirable characteristics of strength, etc., shows a color of about 99, which 'corre sponds to the majority of bleached sulphite fibercommercially on the market.

lVe have discovered that it is possible to produce what may be termed a superbleached pulp, having a high degree of whiteness without loss of or injury to the other desirable qualities and characteristics of cellulosic materials, by subjecting pulp (bleached in the'ordinary way as by the use of an alkaline hypochlorite) to the action of chlorine, either free or in solution in water. Our process, therefore, involves two stages, namely that of bleaching the stock in the ordinary way until a product is produced showing a color of from say 95 to 99, and then super-bleaching it by subjecting the bleached stock to the'action of chlorine in order to remove the remaining traces of ligneous or other coloring matter. A stock, bleached to a point where it shows a color number of 95 to 99, tests high in strength, has a relatively low coeflicient of shrinkage in drying, a high percentage of alpha cellulose and relatively low percentage of alpha cellulose and relatively ow percentages of beta and gamma cellulose, as: suming, of course, that the stock has not been injured in the initial liberation of the fiber by the process of digestion. By treating such a stock with a small quantity 0t chlorine under proper temperature conditions and for a limited period of time, the stock may be super-bleached and whitened to a point where it shows practically no traces of color with practically no diminution in strength and with but silght variations in the relative proportions of the alpha,'beta and gamma cellulose. ,y

In carrying on the first stage of the process by which bleached pulp isproduced, the initial unbleached fiber is, by the use of the usual apparatus (e. g., pulp thickeners), brought to a stock concentration varying from 3% to 17%, this concentration depending upon the particular apparatus which is subsequently used for bleaching. Ordinarily a calcium hypochlorite solution, containing about 32 grams of available chlorine per liter, is added'to the stock in sufficient quantity to correspond to from 10% to 25% of bleach powder based on the quantity otdry pulp stock. The mixture is maintained in agitation for a sufiicient time to allow complete reaction, after which the stock is washed free of the reaction products, a small amount of antichlor being added when necessary to remove the remainingtraces of the chlorine products. To prevent too rapid an oxidation of the lignin content, the temperature is regulated, since a temperature above 100 F usually causes the formation of oxycellulose and thereby affects the strength of the finished bleached stock. The bleached stock, which is thus produced, rep resents the ordinary commercial bleached pulp having a color number of approximately 97 to 99, a copper number of about 2.9 to 3.5 with a strength of 120 expressed in arbitrary units. This bleached stock is now subjected to the second stage of the process, namely to the action of the chlorine by which it is super-bleached. As previously stated, the chlorine may be used either as a gas which is brought into contact with the wet stock, or it may he used as an aqueous chlorine-solution which is added to the wet stock in suflicient quantities to give the desired result. These two ways of introducing the chlorine into the stock give substantially similar results. In practice, it is usually easier to control the amount of the chlorine employed for a given quantity of pulp by forming it into a solution. \Vhere gaseous chlorine is employed, it may be either concentrated or it may be diluted with air. By the addition of chlorine equivalent to from 0.2% to 0.5% by weight of the original air-dried stock, we are able to increase the color number to 102 to 103 without sacrificing in any way the strength and other desirable qualities of the finished stock. The reaction usually takes from 15 to 20 minutes. The pulp, as a result of the treatment, assumes a very pale yellowish tint, doubtless due to the soluble reaction products resulting from the treatment. A bisulphite solution is then added to the pulp in sufiicient quantites to neutralize any resulting hydrochloric acid and to combine with any remaining traces of chlorine. The bisulphite may be added either in the form of sodium or calcium and usually contains an excess of free sulphurous acid. The

stock, of course, is agitated, and, after the addition of the bisulphite, sufiicient time isallowed to elapse for effecting a complete mixture,-and the stock is then thoroughly washed with clean water and is ready for screening, rifliing and other subsequent op erations. It is not necessary that the stock should be washed at the end of the first stage of operations in carrying out our process, as the pulp may be treated with chlorine before the washing takes place. A saving of chlorine and a somewhat higher color is oh:

tained, however, if the stock is washed at the end of the first stage of operations before the chlorine is added. Instead of subjecting thestock in the second stage of the operations to a single acid treatment, it is quite evident that the stock may be first treated with chlorine, then washed, then given .a second treatment of chlorine, followed by a final washing, the addition of the sulphite for neutralization of the hydrobleached pulp after the first stage in the bleaching operation and the second column shows the tests of the same pulp after being subjected to the chlorine treatment.

. Supersi bleached Test. z pulp,

stage second stage.

Strength 120 12 slowness 10' 35" 9 20 @hrinkage 20. 8 20. 2 Per cent alpha cellulose. 87. 7 S9. 5 Per cent beta cellulose 2. 1 2. 2 Per cent gamma cellulose 10.2 S. 3 Per cent total cellulose. 98. 92 99. 42 Copper number 2. 95 3. 25 (oor.....- 99 101+ acid (HOCl), so that undoubtedly a portion of the lignin content remaining in the stock is oxidized. On the other hand, however, there is a perceptible yellowish tint ,in the stock after the treatment which would indi cate that a chlorination of certain of the stock constituents hadtaken place, so that in all likelihood there is an action of the free chlorine directly on the stock in addi tion to the oxidation resulting from the presence of the hypochlorous acid.

. We are aware that it has beenproposed primarily to bleach unbleached pulp .with a chlorine solution in which the resulting hypochloric acid is neutralized by a base such as sodium or calcium carbonate. The action of such a bleaching solution, however, upon unbleached stock is exceedingly harsh or else an excess quantity of the base must be employed. In any case, however, such a treatment'will not give a stock having the exceptional color produced by the. process herein described without destroying or injuring the strength and other desirable characteristics of the stock.

What we claim is: 1. The herein'described process of uperbleaching cellulosic material, which "com to bleach and whiten said material without markedly decreasing the strength thereof, washing said material and removing the reaction products, and then subjecting such material to the action of chlorine whereby the said material is super-bleached to a greater whiteness without substantially decreasing the strength or i affecting the 'other characteristics of such ccllulosic material.

2. The herein described process of bleaching cellulosic material, which comprises two. stages of operations, namely, first subjecting the unbleached material to the usual bleaching action of an alkaline hypochlorite to initially bleach the same, and removing the products of? reaction, and second, subjec-ting the bleached material to the action of chlorine to increase the whiteness of said material without any substantial change in the strength or beating qualities thereof.

materially 3. A process of super-bleaching bleached cellulosic material, which. comprises subjecting said bleached material to the action of chlorine to increase the whiteness of said material without any substantial change in the strength or beating qualities of the material.

4. A process of super-bleaching bleached cellulosic material, comprising the steps of removing the reaction products from the first bleaching operation, and then treating the material with chlorine to super-bleach the bleached material without substantial change in the strength or beating qualities of the material.

5. The herein described proces of bleaching cellulosic material, which comprises the following stages of operations, namely, subjecting the unbleached material to the bleaching action of an alkaline hypochloritc to initially bleach the same as ordinarily, washing the material to free it of the bleaching agent and the products of the reaction, and treating the bleached pulp with an aqueous solution of chlorine with ambleached material to. the bleaching action of an alkaline hypochlorite to initially bleach'the same and washing the material to free it of the bleaching agent and the products of the reaction, and, second, super-bleaching the bleached pulp by treating it with an aqueous solution of chlorine, with the chlorinein the proportion of about 0.2% to 0.5% by weight of the original airdried cellulosic material, adding an antichlor to the mixture and finally washing such mixture to free the cellulosic material from impurities.

7. A super-bleached cellulosie pulp product containing approximately 89% alpha cellulose and a roxnnntelv 2'4 beta cellulose and having approximately the Same strength and beatlng qualltles as the same pulp when subjected to the ordinary commercial bleaching operation, but having an increased Whiteness over said last mentioned pulp. 10 In testimony whereof we have afiixed our ELWOOD EBI E. GEORGE A. RICHTER.

signatures. 

